1. Field of the Invention
This invention relates to a power converter control system, and more particularly to a control system for pulse-width modulation-controlled (PWM control) converters which convert AC power to DC power and PWM control inverters which convert DC power to AC power.
2. Description of the Related Art
FIG. 13 shows a schematic diagram of one phase (the U phase) of a prior art PWM control inverter. FIG. 13(a) shows the main circuit of a power converter.
In FIG. 13(a), V.sub.d1, V.sub.d2 are DC voltage sources, S.sub.1, S.sub.2 are self-turn-off devices, D.sub.1, D.sub.2 are free-wheeling diodes, LOAD is a load and CT.sub.u is a current detector. Also, FIG. 13(b) shows the control circuit for the power converter. Here, C.sub.u, C.sub.1 are comparators, G.sub.u (S) is a current control compensation circuit, PWMC is a pulse-width modulation control circuit, TRG is a carrier wave generator, SM is a Schmitt circuit and GC is a gate circuit. Here, load current I.sub.u is detected by current detector CT.sub.u. This is compared with a current command value I.sub.uO by comparator C.sub.u and a deviation .epsilon..sub.u =I.sub.uO -I.sub.u is found. This deviation .epsilon..sub.u is amplified by current control compensation circuit G.sub.u (S) to generate a voltage command value e.sub.u =G.sub.u (S).multidot..epsilon..sub.u, which is inputted to pulse-width modulation control circuit PWMC.
In pulse-width modulation control circuit PWMC, carrier wave generator TRG generates a triangular wave X. This is compared with inputted signal e.sub.u by comparator C.sub.1, and a gate signal g.sub.u is produced via Schmitt circuit SM. Gate circuit GC inputs this gate signal g.sub.u and produces gate signals g.sub.1 and g.sub.2 and for self-turn-off devices S.sub.1 and S.sub.2. The composition of this circuit explained later.
The example described above is an example for single phase output inverters, e.g. U phase, only. In the case of 3-phase output inverters, apart from this, the V and W phase circuits also have similar compositions.
In the following explanation, voltage command value e.sub.u and its related values K.sub.MAX, X, E.sub.a and E.sub.b use normalized values.
FIG. 14 is a time chart to illustrate the operation of PWM control circuit PWMC in FIG. 13. That is to say,
When e.sub.u .gtoreq.X, g.sub.u =1 and S.sub.1 : ON (S.sub.2 :OFF).
When e.sub.u &lt;X, g.sub.u =0 and S.sub.1 :OFF (S.sub.2 :ON)
At this time, when DC power source voltages V.sub.d1, V.sub.d2 are taken as V.sub.d1 =V.sub.d2 =V.sub.d /2, inverter output voltage V.sub.u becomes
V.sub.u =+V.sub.d /2, when S.sub.1 is ON (S.sub.2 is OFF): and
V.sub.u =-V.sub.d /2, when S.sub.1 is OFF (S.sub.2 is ON).
The mean value MV.sub.u of inverter output voltage V.sub.u (shown by the pecked line) becomes a value proportional to input signal e.sub.u. Therefore, this input signal e.sub.u becomes the inverter voltage command value.
When I.sub.uO &gt;I.sub.u, deviation .epsilon..sub.u =I.sub.uO-I.sub.u becomes a positive value, and voltage command value e.sub.u increases. Therefore, inverter output voltage V.sub.u increases in proportion to e.sub.u and increases load current I.sub.u.
Conversely, when I.sub.uO &lt;I.sub.u, deviation .epsilon..sub.u =I.sub.uO -I.sub.u becomes a negative value and voltage command value e.sub.u decreases. Therefore, inverter output voltage V.sub.u decreases and this decreases load current I.sub.u.
Control is exercised so that, finally, I.sub.u =I.sub.uO. When current command value I.sub.uO is changed as sine wave form, load current I.sub.u also is controlled to follow this, and a sine wave current can be supplied to load LOAD.
In this way PWM control inverters can obtain output voltage V.sub.u proportional to voltage command value e.sub.u. They are therefore widely used in the driving systems of AC motors as variable voltage variable frequency power sources.
However, prior art PWM control inverters have the following problem.
Self-turn-off devices such as gate turn-off thyristors (GTO) are used as devices which compose the inverter. However, in order to protect these self-turn-off devices (hereafter, "devices"), well-known, snubbey circuits are connected in parallel to the devices. When the device is temporarily switched ON in order to initialize (discharge) the capacitor of this snubber circuit, a constant-time ON state must be maintained for the device. Also, minimum ON, OFF times are determined by the characteristic of the device itself, and the pulse-width of the gate signal is supplied to satisfy this.
In FIG. 14, +k.sub.MAX and -k.sub.MAX express an upper limit value and a lower limit value of voltage command value e.sub.u. Output voltage V.sub.u, which is proportional to this voltage command value e.sub.u within the limits +k.sub.MAX .gtoreq.e.sub.u .gtoreq.-k.sub.MAX can be generated.
When e.sub.u =+k.sub.MAX, the period of gate signal g.sub.u =0 becomes .DELTA.t, and this satisfies the minimum ON time of device S.sub.2 (the minimum OFF time of device S.sub.1). Similarly, when e.sub.u =-k.sub.MAX, the period of gate signal g.sub.u 1 becomes .DELTA.t, and this satisfies the minimum ON time of device S.sub.1 (the minimum OFF time of device S.sub.2).
When e.sub.u &gt;+k.sub.MAX or e.sub.u &lt;-k.sub.MAX, the period of gate signal g.sub.u =0 or g.sub.u =1 becomes shorter than .DELTA.t. Therefore, the minimum ON or OFF times of the devices cannot be satisfied. Because of this, the voltage command value e.sub.u is controlled to be within the limits of +k.sub.MAX .gtoreq.e.sub.u .gtoreq.-k.sub.MAX by providing a limiter circuit or the like (not illustrated).
For example, when the carrier frequency f.sub.c is taken as f.sub.c =500 Hz, cycle T of triangular wave X becomes 2 msec, and in order to satisfy the minimum ON time (or minimum OFF time) .DELTA.t=200 .mu.sec, k.sub.MAX =0.8. That is to say, in this case the utilization factor of the inverter is 80%, and the remaining 20% is redundant.
Therefore, a greater inverter capacity had to be prepared for the portion by which the utilization factor was reduced. Thus, the prior art PWM control inverter was an uneconomic system.